Epistatic interactions between subtypes of KIR3DL1 and its ligand, HLA-B, predict for differential outcomes in patients with HIV (Martin et al., Nature Genetics 39:733-740 (2007)) and undergoing hematopoietic stem cell transplantation (HCT) (Giglio et al. (November 2012)). Functional groups of KIR3DL1 predict for high- and low-inhibitory interactions with HLA-Bw4 and correlate with clinical outcomes in HIV and leukemia patients undergoing hematopoietic cell transplantation (HCT) (Martin et al., Nature Genetics 39:733-740 (2007); Giglio et al. (November 2012)). The 59 known inhibitory alleles have previously been classified into three subgroups based on the density with which they are expressed on the surface of NK cells (Gardiner et al., Journal of Immunology 166:2992-3001 (2001); Pando et al., Journal of Immunology 171:6640-6649 (2003); Yawata et al., J Exp Med 203). A group of KIR3DL1 alleles that encode the activating KIR3DS1 molecules accounts for an additional 12 alleles of KIR3DL1. While HLA-B alleles can be readily identified, a simple and cost-effective protocol for KIR3DL1 allele assessment is yet to be developed in order to translate the research findings into clinical practice.